Single positive voltage supplies are relatively common for audio amplifiers in mobile devices powered by a battery. In the past, an audio amplifier powered by a single positive voltage supply provided a relatively limited dynamic range for the audio signal to drive speakers, or headphones. In particular, the maximum peak-to-peak swing of the audio signal was relatively limited or clamped between the power supply's positive output supply rail (VDD) and ground. See FIG. 1A for graph 100 of a Line_In audio signal whose potential dynamic range is clamped between the VDD rail and ground.
To improve the dynamic range for an audio signal, some audio amplifiers powered by a single positive voltage supply have also included a charge pump circuit to synthesize a negative voltage supply rail (VSS). The charge pump circuit can be arranged to provide the VSS voltage rail with a magnitude that can be relatively equivalent to the magnitude of the positive voltage supply rail (VDD). By providing relatively equivalent negative and positive voltage supply rails (VSS and VDD) to the audio amplifier, the maximum peak to peak swing of the audio signal and corresponding dynamic range can be relatively twice as large as a somewhat similar audio amplifier that only enables the audio signal to swing between the VDD rail and ground.
FIG. 1B illustrates graph 110 of a Line_In audio signal that can swing between the VDD rail and the VSS rail. Also, the magnitudes of the VDD and VSS rails illustrated in FIG. 1B are relatively the same, and these magnitudes are also relatively similar to the magnitude of the VDD rail shown in FIG. 1A. Although a sinusoidal Line_In signal is shown in FIGS. 1A and 1B, the invention is not so limited. Rather, the Line_In signal can be a digital signal, square wave signal, triangular wave signal, or any other type of waveform, without departing from the spirit and scope of the invention.
FIG. 1C illustrates graph 100 of an audio signal whose potential dynamic range is clamped between the VDD rail and ground. Also, FIG. 1D illustrates graph 130 of an audio signal that can swing between the VDD rail and the VSS rail. Also, the magnitudes of the VDD and VSS rails illustrated in FIG. 1D are relatively the same, and these magnitudes are also relatively similar to the magnitude of the VDD rail shown in FIG. 1C.
In many electronic devices, it has become more common to connect Line_In audio signals from other electronic devices to the output of an audio amplifier. However, when an audio amplifier with a charge pump circuit is placed in a shut down mode to isolate these Line_In signals from the audio amplifier's components, it is a relatively common practice to also disable the charge pump circuit which turns off the VSS voltage rail. Without the presence of the VSS voltage rail, the dynamic range of the Line_In audio signal from another electronic device coupled to the amplifier's output can be limited while the audio amplifier is in a shut down mode.